For example, a logic circuit such as a microcontroller cannot directly supply current and voltage required for driving a power switch such as an insulated-gate electrode transistor (IGBT) having an insulated control terminal-electrode, a transistor, or a metal-oxide-semiconductor field-effect transistor (MOSFET). Due to this, in a power electronic device assembly, a single- or multi-driver circuit is disposed between the logic circuit and the power switch. Meanwhile, the driver circuit is provided to control required current and voltage. On the other hand, a structure in which a protective and safety function is integrated in the driver circuit belongs to the prior art. Recently, most driver circuits are converted based on an integrated circuit (IC).
A protective- and safety-function which is mainly used in a driver-IC is a “Miller-clamp” function of a short circuit that is configured to selectively short-circuit a control input (gate, base) and a reference leg (emitter, source) of the power switch. In this regard, when the power switch is not in operation, the input (gate, base) of the power switch is actively transmitted by the driver to a potential far below a switching-on threshold value of the power switch. Thereby, first of all, the power switch is undesirably switched on by charge of a parasitic capacitance. Here, the parasitic capacitance may be a so-called Miller-capacitance which is generated between the input (gate) and a collector of the power switch in the case where the IGBT is used as the power switch, and which is generated between the input (gate) and an output (drain) of the power switch in the case where the MOSFET is used as the power switch.
The risk of switching-on through the parasitic capacitance of the power switch is present in an assembly that particularly includes a power switch for high-current and high-voltage with a high switch frequency. A prior art aims to embody a Miller-clamp using a switch integrated in the driver-IC. Such a switch is typically embodied in the IC by an N-channel-FET (field effect transistor) or an NPN-transistor.
Connection of the switch, in other words, activation of the Miller-clamp, is embodied only when a positive (+) voltage is applied to a transistor disposed in the IC. The reason for this is because a positive supply voltage is required for connection of the N-channel-FET or the NPN-transistor. If no or an excessively small amount of supply voltage for operating the driver-IC is provided, the protective function of the Miller-clamp is also not operated. This occurs every time an inverter configured to supply power to a driver for an upper power switch, that is, a high-side-power switch driver, through a bootstrapping circuit, starts. Bootstrapping is generated by connection of a driver supply voltage for the high-side driver to a lower power switch, that is, a low-side-switch, of a half bridge. Due to this, even while the inverter is already actively operated, the protective function integrated in the driver IC may not act due to undervoltage. This condition may cause damage to a power unit.